1. Field of the Invention
The invention relates generally to semiconductor devices having resistors and more particularly to semiconductor devices constructed for operation in the microwave frequency ranges.
2. Description of the Prior Art
High power microwave silicon transistors involve a large number of elements each producing a high level of current. Inevitably, resistive losses lead to Joule heating of the silicon. In bipolar transistors, the current is proportional to reciprocal temperature; that is the current increases quickly as the temperature increases. Because of this characteristic of bipolar transistors, thermal runaway eventually ensues leading to the ultimate failure of the device. The onset of thermal runaway can be delayed by adding a ballast resistor in series with each element or group of elements (for example, emitter fingers in an interdigitated design). These ballast resistors which provide negative feedback to the emitter-base junction, preventing thermal runaway, may be formed in a number of ways. In one approach, a material, such as boron, may be diffused into the silicon. In this approach, the resistivity of the diffusion material is utilized as the resistor. However, the diffused silicon also has a negative coefficient of resistivity, reducing the feedback effect of the ballast resistor. It would be advantageous to use a ballast resistor having a positive coefficient of resistivity. In another approach, a relatively large segment of a metal such as aluminum is placed between the current elements. Aside from the size drawbacks, this approach has the drawback of the metal segment contributing parasitic capacitance.
A preferred type of ballast resistor is the thin film resistor. The thin film ballast resistor is created by applying a thin film of material having a selected resistivity between current elements. For all methods of forming resistors, control is an issue. For thin film resistors, it is often difficult to effectively control the film thickness and linewidth, thus the actual resistance of the thin film resistor may deviate from the desired value.
The particular value of the ballast resistor chosen in such applications is important. In addition to increasing reliability and ruggedness, this resistor affects the power gain and collector efficiency of the transistor. The accuracy of the value of the resistors is thus important.
Due to variations in film thickness, resistor pattern dimensions and the resistivity of the resistor material, precise control of the resistance of the ballast resistors is difficult to achieve. The art has attempted to correct or alter the value of the resistors after the resistors have been applied. This process is called trimming the resistors. Typical trimming techniques have involved physically removing or disconnecting resistor material with a laser. Size and cost considerations make this technique extremely impractical when dealing with a large number of resistors. Thus, it would be advantageous to provide a practical means for fine tuning the resistance of the ballast resistors after the resistor material has been applied.
Aluminum is commonly used as the primary contact metal for silicon integrated circuits. For high power devices where electromigration is a serious problem, gold may be preferred. For either case, a barrier metal is needed to prevent diffusion of the primary contact metal into the underlying silicon or silicideo. Thus, it would be advantageous to provide a method of simultaneously providing the barrier metal and the ballast resistors in a single layer of material deposition.